The significance of humification pathway as promoted by metallic oxides has previously been well investigated using a model system containing humus precursors. Metallic oxides, co-existing with organic matter in nature, are capable of promoting humification of organic matter to different degrees. This study is an investigation into the role of ferric oxide in the process of humification, based on the degree of darkening for humus precursors. Glucose (Glu), glycine (Gly) and catechol (Cat) were introduced as humification model precursors, and the role of ferric oxide in different periods of humification was discussed by characterizing humification degree in terms of dissolved organic carbon, ratio of Fulvic acids (FA) to Humic acids (HA), and E 600 . Valence changes of Fe were analyzed using X-ray photoelectron spectroscopy. The results showed that ferric oxide was effective on the conversion from FA to HA, and the effectiveness grew as the ferric oxide concentration increased. A higher pH value was more conducive to form dark substances. Amounts of Fe with various valences changed remarkably after humification, suggesting that ferric oxide was involved in the humification reaction.

Ultraviolet-visible and e.s.r. spectroscopies have been used to investigate the reactions of humic substances, extracted from swamp waters, with manganese(VII) and with chromium(VI). Potassium permanganate reacts quickly with humic substances to form manganese(II) derivatives. Phenolic groups are probably involved as a similar reaction occurs with simple phenols such as phenol and catechol. Manganese(II) interacts directly with carboxylate anionic groups in humic substances. E.s.r. spectroscopy shows that manganese(II) forms outer-sphere complexes with humic substances. E.s.r. spectra show that chromium(VI) is reduced to chromium(V) and then to chromium(III) in the presence of humic substances. The rate reduction of chromium(VI) depends on the concentration of humic substances and on pH, temperature, and u.v.-visible radiation. At pH 3 the rate of formation of chromium(V) is about 100 times faster than the rate of its reduction. At neutral pH and at room temperature the chromium(V) species is stable for a long period.